1. Field of the Invention
The invention relates to stators of compressors with vanes of variable angle.
2. Summary of the Prior Art
Stators of this type are constituted by vanes mounted with the facility of rotation about their longitudinal axis within the casing of the compressor, their angle of incidence with respect to the axis of the engine being controlled by a mechanism external to the casing, comprising, for example, a synchronizing ring of which the motion is transmitted to the pivots at the tips of the blades through the intermediary of linkages.
At their inner ends, nearer to the axis of the engine, these vanes are generally interconnected by an inner ring divided into a plurality of sectors of which the upper face defines the interior wall of the flow path and which supports sealing means such as a fluid-tight gasket cooperating with the lips of a labyrinth seal rigid with the rotor in order to restrict the internal flow losses between the downstream and upstream of the stator stage.
The invention relates in particular to the connection point of the roots of the vanes to the internal vane ring. This connection is provided by means of a root pivot within a bearing bush located in a radial bore of the ring; in order to enable free expansion of the ring during operation, the latter has a certain freedom for sliding along the pivots of the vane roots.
The degree of clearance between the lips and the abradable gasket is critical for the efficiency and the performance of the engine. On the assumption of achieved tolerances at 100%, the radial location and the rigidity of the ring are ensured by the radial disposition of the vanes. The clearances are thus determined with precision. In practice, the vanes may acquire, during assembly, an inclination in a plane transverse to the axis of the engine, which is more or less pronounced. Furthermore, the clearance between the pivot and pin is not zero; this leads to a degree of scattering in the radial position of each sector which is not compensated when these sectors are interconnected by fluid-tight and wear seal supporting rails because the rigidity of the assembly is inadequate. Thus, the ring may deviate from a true circle and, its centering with respect to the rotor may be defective. Furthermore, during operation the end positions of the sectors of the rings are difficult to estimate; in the absence of any abutment especially between the platforms of the vanes and the sectors of the ring, the inner portions of the vanes may come into contact with the wall of the ring with, as a consequence, possible damage to the vanes when they are controlled to turn about their axes by the control system.
A first improvement consisted in the reduction of the number of sectors constituting the ring and the rail in order to limit the effects resulting from splitting into parts and to introduce at the tip of each blade below the control lever a packing piece limiting the radial position of the vane, the thickness of this packing piece being determined as a function of the residual clearance at the point of attachment. Finally, by locating a distance piece serving as a stop between the pivot of the root and the rail, it is possible to limit the variations in the radial position of the sectors. Nevertheless, apart from difficulties in realising by this method satisfactory centring of the ring with respect to the rotor its practical use complicates maintenance operations because of the risk of inversion of the packing-piece or distance piece during reassembly.
There is known, furthermore, from U.S. Pat. No. 3,079,128 an arrangement where the root pivot is terminated in a screw thread, in such a manner that the sector of the ring can be radially locked between a nut screwed onto the pivot and the platform of the vane. Such a construction gives rise to the disadvantage that it is not capable of adjustment in the position of the sector of the ring thereby necessitating the repositioning in place, as a result of wear of the fluid-tight and wear seal; furthermore, the ring has no freedom of sliding motion enabling the accommodation of differential expansions during operation.
GB-A No. 749 577 describes an internal ring assembly of a stator with the possibility of adjustment. The vanes are secured to the ring by means of a pin mounted eccentrically within an axial bore of the roots of the vane through the intermediary of a sleeve having inner and outer eccentric axes. It is possible by these means to effect an adjustment in the position of the ring with respect to the vanes and to the rotor. Unfortunately, such a system cannot be applied to a stator of which the vanes have adjustable incidence.
One object of the present invention is therefore to limit variations in the radial position of the ring in a compressor stator having variable angle vanes.
Another object of the invention is substantially to eliminate risk of contact between the inner parts of the vanes and the internal ring.
A further object is to facilitate the control of the position of the ring in such a stator assembly.